1. Field of the Invention
The present invention relates to a process for continuously producing an aromatic polyimide film. More particularly, the present invention relates to a process for continuously producing an aromatic polyimide film with an excellent quality and productivity by means of of extrusion casting of a dope solution of the aromatic polyimide having a relatively high viscosity through an extruder provided with a T-die.
2. Description of the Prior Art
It is known that an aromatic polyimide having a high molecular weight and a high content of imide structure can be produced by carrying out successively an operation for polymerizing an aromatic tetracarboxylic acid or its functional derivative, for example, pyromellitic dianhydride with an aromatic diamine and another operation for converting the resulting polymerization product, that is, a polyamic acid, into the corresponding polyimide resin. It is also known that this type of aromatic polyimide resin is suitable for producing various shaped articles, for example, film and sheet, and the resultant shaped aromatic polyimide resin articles exhibit an excellent heat-resisting property and superior mechanical strength.
However, it is known that the pyromellitic acid type aromatic polyimide resin having a high molecular weight and a high content of the imide structure neither melt even at a high temperature of 300.degree. C. or more, nor one dissolved in any organic solvent. Accordingly, in order to produce a film from the pyromellitic acid type aromatic polyimide resin, a polyamic acid is prepared from pyromellitic dianhydride and an aromatic diamine in an organic polar solvent, the resultant solution of the polyamic acid in an organic polar solvent is spread on a peripheral surface of a metal drum or an upper surface of a metal plate to form a thin film of the solution having a uniform thickness, the film of the solution is subjected to an evaporation procedure of the solvent at an elevated temperature to eliminate the solvent from the film, the evaporation undesirably causing the polyamic acid to be partially imidized, and, finally, the film of the partially imidized polyamic acid is heated at an elevated temperature so as to produce a completely imidized (and dehydrated) aromatic polyimide film.
The above-mentioned known method for producing the aromatic polyimide film is disadvantageous in that in the elimination of the solvent from the film of the polyamic acid solution, the polyamic acid is undesirably partially imidized; it is necessary to convert the polyamic acid film to a polyimide film by completely imidizing (dehydrating) the polyamic acid; the polyamic acid solution is unstable in storage. Therefore, it is very difficult to produce a high performance aromatic polyimide film with a satisfactory reproducibility by the above-mentioned conventional method.
In recent years, in the preparation of aromatic polyimide films it has been proposed to use a solution of aromatic polyimide uniformly dissolved in an organic polar solvent in a high concentration thereof.
For example, Japanese Unexamined Patent Publication No. 50-113597 (1975) discloses a solution of an aromatic polyimide resin in an organic polar solvent. The solution is prepared in such a manner that a mixture of a biphenyl tetracarboxylic dianhydride with an aromatic diamine is heated in an organic polar solvent consisting of cresol, a mixture of xylene and cresol or a mixture of N-methylpyrrolidone and xylene, at an elevated temperature, thereby to give a viscous solution. This patent publication also discloses an aromatic polyimide film prepared in such a manner that the polyimide solution is spread on a surface of a copper plate; the thus spread solution on the copper plate is dried at a temperature of about 100.degree. C.; the dried film is baked at a temperature of 200.degree. C., and; the copper plate is then etched out to obtain an aromatic polyimide film.
However, the above-mentioned process contains an impractical procedure of etching out the copper plate. Therefore, this process is inferior in productivity and useless for industry.
Furthermore, it is well known that generally, a polymer film can be produced by naturally spreading a low viscosity solution of the polymer is a solvent on a peripheral surface of a drum or an upper surface of a flat plate, to provide a film of the polymer solution and by gradually evaporating away the solvent from the polymer solution film. Therefore, from the above-mentioned prior arts, it is expected that the above-mentioned aromatic polyimide solution having a high viscosity can be converted into a solid film by the solution-spreading method.
However, an aromatic polyimide having a high molecular weight and a high melting point can be dissolved in high concentration only in certain types of phenol compounds, and the resultant solution exhibits a high viscosity. Therefore, it is impossible to spread the highly viscous solution of the aromatic polyimide on the peripheral surface of the drum or the upper surface of a flat surface at an ambient temperature and to produce a thin film of the solution at the ambient temperature. In order to produce the film from the coventional aromatic polyimide solution by using the solution-spreading method, it is necessary to decrease the viscosity of the aromatic polyimide solution to an extent that the solution can be naturally spread, by elevating the temperature of the solution to a level close to the boiling point of the solvent in the solution. This elevated temperature of the solution causes the solvent in the spread solution film to by vigorously evaporated and, sometimes, results in the formation of bubbles in the film of the solution. In this case, it is impossible to stably produce an aromatic polyimide film which has a satisfactory quality and is, therefore, practically useful.
Under the above-mentioned circumstances, it was strongly desired in the aromatic polyimide industry to provide a continuous process for stably producing aromatic polyimide film from a solution of the polyimide having a high viscosity without causing any difficulties which are inherent in the conventional processes.